Alex outlined a Software/Hardware system that allows you to use current or older HF gear with an SDR based replacement for the original mod/demod section of the rig.

Essentially what he's doing is tapping into the receiver IF and using SDR techniques to provide demodulation on receive, and also tapping into the transmitter section to modulate. Couple this with PC software to control the radio and do the mod/demod and you have some of the benefits of SDR applied to older radios.